fix: avoid panic during task cache construction (#5882)
### Description
When hooking up the task graph visitor to the task cache I hit a
[failing debug
assertion](https://github.com/vercel/turbo/blob/main/crates/turborepo-paths/src/absolute_system_path.rs#L179).
This was triggered by the the [join we
perform](https://github.com/vercel/turbo/blob/main/crates/turborepo-lib/src/run/cache.rs#L69)
in the task graph construction.
This PR:
- We join anchored workspace directory instead of the resolved absolute
system path. [This now matches
Go](https://github.com/vercel/turbo/blob/main/cli/internal/runcache/runcache.go#L313)
- Moves this logic into it's own method so it can be tested without an
entire `RunCache` instance
- Removes an unnecessary cloning of the `TaskOutput` object
I'll note that this *probably* isn't what we want to do in the future as
we're joining anchored system paths with globs that will [most
definitely contain relative unix
paths](https://github.com/vercel/turbo/blob/main/cli/internal/nodes/packagetask.go#L38).
On windows this results in globs like `apps\web\.turbo/turbo-build.log`.
I do want to fix this, but since that would be a behavior change in both
Go&Rust, it belongs in its own PR.
### Testing Instructions
Moved code that panicked to it's own method and added a unit test that
would exercise it.
---------
Co-authored-by: Chris Olszewski <Chris Olszewski>
